Method for manufacturing short fiber oriented rubber or synthetic resine and expansion die for extrusion molding

ABSTRACT

A method comprises steps of extruding rubber or synthetic resin mixed with short fiber from a mold opening  6  of sectionally annular and molding a cylindrical body of rubber or synthetic resin with short fiber oriented in predetermined direction. And the mold opening  6  is passed through with an expansion portion  9  whose diameter expands in extrusion direction, and a flow passage width Wi of an inlet portion  6   i  of the mold opening, a flow passage width Wo of an outlet portion  6   o , and radiuses Ri and Ro of the inlet portion  6   i  and the outlet portion  6   o  respectively that are coaxial with the mold opening satisfy a relation of Wo/Wi&lt;Ro/Ri. And value of Ro/Ri is adjusted within a scope of 2≦Ro/Ri≦8 by changing the radius Ri of the inlet portion according to mechanical property in circumferential direction of the cylindrical body to be molded, so that orientation of the short fiber is adjusted.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing short fiber oriented rubber or synthetic resin wherein rubber or synthetic resin mixed with short fiber is extruded from a sectionally annular mold opening and a cylindrical body with short fiber oriented in a predetermined direction and an art of an expansion die for extrusion molding that has the sectionally annular mold opening.

BACKGROUND OF THE INVENTION

With respect to an art of orienting short fiber mixed in rubber or synthetic resin, known is a method for extruding rubber or synthetic resin mixed with short fiber, from an expansion die for extrusion molding having a sectionally annular mold opening. As for the expansion die for extrusion molding, for example the next one is included. The mold opening comprises an inlet opening, a middle opening (expanded portion), and an outlet opening, wherein sectional area of the outlet opening is formed larger than that of the inlet opening, an flow passage width of the inlet portion is smaller than that of the outlet portion, the flow passage width of the outlet portion is less than that of the middle opening, and the middle opening is provided with expansion opening portion having expanded flow passage width (e.g. Japanese Patent Publication No. H6-9847 (Pages 2 to 4, FIGS. 1 to 5).

When rubber or synthetic resin mixed with short fiber is extruded from this expansion die for extrusion molding, orientation rate of the short fiber in the circumferential direction and the radius direction in the middle opening increases so that a cylindrical body where short fiber is oriented at predetermined rate in the extrusion direction and in the circumferential direction and the radius direction respectively.

Thus formed cylindrical body is cut open in the longitudinal direction to form into a sheet, and subsequently cut in the predetermined length, and it is used for bottom rubber of a transmission belt, buffer material, seal material, shoe sole, floor material, caterpillar cover material, etc. Since the sheet material mixed with short fiber has high strength and modulus in the orientation direction, formed sheet material (i.e. cylindrical body) has different balance in orientation rate in each direction according to application. For example, in the case of the sheet material used for bottom rubber of V belt, where lateral pressure resistance against load laterally received is required to prevent deformation, the cylindrical body having high orientation rate in circumferential direction is formed and this is cut open in the longitudinal direction to the predetermined length and cut in the width direction at predetermined intervals. Thus the sheet material with short fiber oriented in the width direction can be obtained by cutting open the cylindrical body. Therefore the sheet material can be more efficiently molded compared with the conventional calendar molding method where the cut sheet materials are connected at width direction end portion.

Meanwhile, when the rubber or synthetic resin mixed with short finer is extruded from the above described expansion die having the mold opening, the short fiber is apt to be oriented in the circumferential direction. However, it is more apt to be oriented in the extrusion direction and in the radius direction depending on shapes of the mold opening and orientation rate in the circumferential direction is lowered and therefore it is difficult in some cases to form the cylindrical body that satisfies mechanical property in the circumferential direction.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for manufacturing short fiber oriented rubber or synthetic resin and an extrusion molding die head, wherein cylindrical body having excellent mechanical property in a circumferential direction can be easily molded by circumferentially orienting short fiber efficiently.

According to a first aspect of a method for manufacturing short fiber oriented rubber or synthetic resin, the method comprises steps of extruding rubber or synthetic resin mixed with short fiber from a mold opening that is sectionally annular, molding a cylindrical body of the rubber or synthetic resin with the short fiber oriented in a predetermined direction, and passing through the mold opening an expansion portion with a diameter expanding in extrusion direction, wherein a flow passage width Wi of an inlet portion of the mold opening, a flow passage width Wo of an outlet portion, and radiuses Ri and Ro of the inlet portion and the outlet portion respectively that are coaxial with the mold opening satisfying a relation of Wo/Wi<Ro/Ri, and orientation of the short fiber is adjusted by adjusting a value of Ro/Ri within a scope of 2≦Ro/Ri≦8 according to mechanical property in circumferential direction of the cylindrical body to be molded.

According to this method for manufacturing the short fiber oriented rubber or synthetic resin, the short fiber is apt to orient in circumferential direction (Y direction) of the cylindrical body because the relation of Wo/Wi<Ro/Ri is satisfied. And the short fiber can be efficiently oriented in Y direction by adjusting the value of Ro/Ri within a scope of 2≦Ro/Ri≦8.

That means, Z direction orientation rate of the short fiber (thickness direction of the cylindrical body) depends on Wo/Wi, and Y direction orientation depends on the expansion ratio Ro/Ri. Therefore, if Ro/Ri is larger than Wo/Wi, the short fiber is more easily oriented in Y direction than in Z direction. And if Ro/Ri is less than 2, the short fiber can not be exclusively oriented in Y direction. If Ro/Ri exceeds 8, increase tendency of the y direction orientation rate is extremely less. This is because Y direction orientation rate to the expansion ratio shows a value close to arctangent of circumferential draw rate (or expansion ratio) of the rubber or synthetic resin in the mold opening.

Therefore, when the rubber or synthetic resin including the short fiber is extruded through the mode opening with expansion ratio where Ro/Ri is adjusted within a scope of 2≦Ro/Ri≦8, the short fiber is circumferentially oriented efficiently, so that the cylindrical body excellent in mechanical property in circumferential direction can be easily molded.

Since the method for manufacturing short fiber oriented rubber or synthetic resin according to the above described first aspect can easily mold the cylindrical body having excellent mechanical property in Y direction, productivity of products manufactured using this cylindrical body is increased. Besides since orientation of the short fiber is adjusted by adjusting the expansion ratio, a various types of products can be easily responded.

According to a second aspect of the rubber or synthetic resin, aspect ratio of the short fiber is 10 to 300.

According to this method for manufacturing the short fiber oriented rubber or synthetic resin, since the mixed short fiber can be effectively oriented in flow direction of the rubber or synthetic resin, the short fiber can be circumferentially oriented more efficiently.

That means, the aspect ratio of the short fiber is less than 10, efficiency in the relation between expansion ratio Ro/Ri and orientation rate. Or the Y direction orientation to the expansion rate Ro/Ri extremely decreases and mechanical property in circumferential direction of the cylindrical body to be molded is short. And when the aspect ration exceeds 300, the short fiber exists in clumps in the rubber or synthetic resin, the cylindrical body of stable quality can not be molded because deformation occurs during vulcanization and strength distribution is uneven. Basically, when diameters of the short fibers are the same, longer one is more possibly oriented and a scope of the aspect ratio 30 to 150 is preferable. As for the short fiber, used are e.g. aramid fiber, polyamide fiber, polyester fiber, and cotton.

Since the above described second aspect of the method for molding short fiber oriented rubber or synthetic resin can easily mold the cylindrical body having excellent mechanical property in Y direction and the cylindrical body to be molded includes short fiber having industrially useful aspect ratio, productivity of the products can be increased while maintaining quality.

A third aspect of the method for molding short fiber oriented rubber or synthetic resin features that a flow passage width of the mold opening is constant.

According to this method for molding short fiber oriented rubber or synthetic resin, the short fiber is difficult to be oriented in Z direction in the mold opening, and therefore the Y direction orientation efficiency of the short fiber increases as a result.

In the method for molding short fiber oriented rubber or synthetic resin according to the above described third aspect, the cylindrical body having excellent Y direction mechanical property can be efficiently molded.

A fourth aspect of an expansion die for extrusion molding comprises a mold opening that is sectionally annular and formed with an inner surface of an outer die and an outer surface of an inner die, and an expansion portion whose diameter expands in extrusion direction being provided to the mold opening, wherein a flow passage width Wi of an inlet portion of the mold opening, a flow passage width Wo of an outlet portion, and radiuses Ri and Ro of the inlet portion and the outlet portion respectively that are coaxial with the mold opening have a relation of Wo/Wi<Ro/Ri, and an expansion ratio Ro/Ri is adjusted within a scope of 2≦Ro/Ri≦8 by changing the radius Ri of the inlet portion according to mechanical property in circumferential direction of the cylindrical body to be molded, so that orientation of the short fiber is adjustable.

According to this expansion die for extrusion molding, Z direction orientation rate of the short fiber depends on Wo/Wi, and Y direction orientation depends on Ro/Ri. Therefore, if Ro/Ri is larger than Wo/Wi, the short fiber is more easily oriented in Y direction. And the value of Ro/Ri is adjusted within a scope of 2≦Ro/Ri≦8 by changing the radius Ri of the inlet portion, thereby capable of efficiently orienting the short fiber in Y direction.

That means, if Ro/Ri is less than 2, the short fiber can not be exclusively oriented in Y direction. If Ro/Ri exceeds 8, increase tendency of the y direction orientation rate is extremely less. This is because Y direction orientation rate to the expansion ratio shows a value close to arctangent of circumferential draw rate (multiple factor or expansion ratio) of the rubber or synthetic resin in the mold opening.

And when Ro/Ri is too large, there are problems as follows. When the expansion ratio Ro/Ri is enlarged by expanding the radius Ro of the outlet portion, a head pressure for extruding the rubber or synthetic resin should be high. Then the inner die receiving the head pressure is required to be structured highly strongly and therefore the design becomes difficult. And a radius of the molded cylindrical body is too large, and when the cylindrical body is cut open in longitudinal direction into sheet, a sheet width is extremely broad, which decreases operability. In addition, if the head pressure is raised as above mentioned in the case of extruding the rubber or synthetic resin with a screw, the revolution rate should be raised, which decreases viscosity of the rubber or synthetic resin due to frictional heat with the screw, thereby decreasing orientation of the short fiber.

On the other hand, when Ro/Ri is increased by decreasing the radius of the inlet without changing Ro (that means, the radius of the cylindrical body is constant and the width is constant in the case of sheet state of the cylindrical body), the flow passage width in front of the inlet portion is extremely narrow. And in the case of structure where the expansion die for extrusion molding makes the spider shaft support the inner die, the radius of the spider shaft should be narrowed, and therefore, which could lead to a condition where strength against head pressure can not be secured.

Therefore, the expansion ratio Ro/Ri is adjustable within a scope of 2≦Ro/Ri≦8 and the short fiber is circumferentially oriented efficiently, so that the cylindrical body excellent in mechanical property in circumferential direction can be easily molded.

The above described fourth aspect extrusion die can stably supply the cylindrical body having excellent mechanical property in Y direction and besides can easily cope with various types of products because the orientation of the short fiber can be adjusted by adjusting the expansion ratio.

According to a fifth aspect of the expansion die for extrusion molding, a flow passage width of the mold opening keeps constant.

This expansion die for extrusion molding can make it difficult that the short fiber is oriented in Z direction in the mold opening, and therefore Y direction orientation of the short fiber can be efficiently changed.

According to a fifth aspect of the expansion die for extrusion molding, the cylindrical body having excellent mechanical property in Y direction can be stably supplied.

According to a sixth aspect of the expansion die for extrusion molding, a center block is provided at least one of the outer die and the inner die in such manner that the center block is partially dividable, and the center block is provided with surfaces that partially forms the inner surface of the outer die, the outer surface of the inner die, and the mold opening, and the radius Ri of the inlet portion is changeable by attaching, removing, or exchanging the center block.

According to this expansion die for extrusion molding, since expansion ratio Ro/Ri is adjustable by attaching and removing, or exchanging the center block, a various types of expansion dies can be molded by a single unit of expansion die.

Since the expansion die for extrusion molding according to the sixth aspect can change expansion ratio by attaching/removing or exchanging the center block, a single unit can supply a various types of cylindrical body of rubber or synthetic resin.

According to a seventh aspect of the expansion die for extrusion molding, a flow passage width W7′ in immediately front of the inlet portion after the center block is removed is not less than 5 times of the flow passage width Wi of the inlet portion.

According to this expansion die for extrusion molding, the rubber or synthetic resin mixed with short fiber is oriented at random in respective directions X, Y, Z without disproportioning orientation rate of the short fiber in one direction in immediate front of the inlet portion so that disadvantageous factor for short fiber orientation in the mold opening is eliminated and the short fiber can be circumferentially oriented efficiently.

The expansion die for extrusion molding of the above described seventh aspect can stably supply the cylindrical body having excellent Y direction mechanical property.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical-cross sectional diagram showing an embodiment of an expansion die for extrusion molding related to the present invention.

FIG. 2 is a partially expanded diagram of a mold opening in FIG. 1.

FIG. 3 is a vertical-cross sectional diagram of the expansion die for extrusion molding in a state that a center block is exchanged in FIG. 1.

FIG. 4 is a vertical-cross sectional diagram of the expansion die for extruding molding in a sate that the center block is removed.

FIG. 5 is a graph showing orientation rate and arc tangent value in Y direction in Tests 1 and 2.

FIG. 6 is a vertical cross sectional diagram showing another embodiment of a center block of an expansion die for extrusion molding related to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the method for manufacturing short fiber oriented rubber or synthetic resin and the expansion die for extrusion molding related to the present invention will be described.

FIG. 1 is a schematic diagram of an expansion die for extrusion molding 1 and a vertical-cross sectional view cut along extrusion direction. FIG. 2 shows partially expanded diagram of a mold opening 6 described later. The expansion die for extrusion molding 1 (also referred to as expansion die 1 hereinafter) is arranged in an extrusion outlet 2 of an extruder, and includes an outer die 3 having an annular cross section and an inner die 5 having a disk shape as principal components. And the outer die 3 is fixed to an end of the extrusion outlet 2, a spider shaft 4 is coaxially supported by the extrusion outlet 2 in such manner that the shaft passes through the extrusion outlet, and the inner die 5 is fixed on the spider shaft 4 in such manner that the shaft passes through a through hole 5 a provided to a center portion of the inner die.

In this expansion die 1, a mold opening 6 that is sectionally annular and molds molding material (rubber or synthetic resin including short fiber) into a cylindrical body is formed between an inner circumferential surface of the outer die 3 and an outer circumferential surface of the inner die 5. The molding material is sent to the mold opening 6 through a flow passage P that is a space provided between the spider shaft 4 and the inner circumferential surfaces of the extrusion outlet 2 and the outer die 3.

In the expansion die 1, a center block 7 that is sectionally annular is provided in such manner that the inner circumferential portion of the outer die 3 is partially dividable with the center block 7. A front side in extrusion direction of an inner circumferential surface of the center block 7 forms part of the inner circumferential surface of the outer die 3 and a surface 7 a for constituting the mold opening 6. A back side in extrusion direction is formed with a taper surface 7 b for narrowing down the flow passage P in extrusion direction. By this taper surface 7 b, pressure of the molding material in the flow passage increases. The center block 7 is fixed in such manner that a screw hole 7 c bored in the outer circumferential portion of the center block 7 in extrusion direction is screwed together with an end of a bolt 8 penetrating the outer die 3.

A flow passage width of the above described mold opening 6 is constantly 3 mm, and relation of Wi=Wc=Wo is established among a flow passage width Wi of an inlet portion 6 i, flow passage width Wc of a middle portion 6 c, and flow passage width Wo of an outlet portion 6 o. In the mold opening 6, an area of the outlet portion 6 o is larger than that of the inlet portion 6 i, and an expansion portion 9 is formed by the inlet portion 6 i and the middle portion 6 c. An expansion ratio of this expansion portion 9 is expressed by Ro/Ri where a radius of the inlet portion 6 i with a shaft center of the spider shaft 4 as a center is Ri and a radius of the outlet portion is Ro, and it is changeable by attaching and removing the center block 7 or exchanging one with a different diameter. This is because the radius direction location of the inlet portion 6 i changes by attaching and removing or exchanging the center block 7, and then the radius Ri changes. With respect this expansion die 1, the case where the center block 7 is attached is expressed by Ro/Ria=7.5 (FIG. 1), the case where a ring-shape center block 10 is exchanged with is expressed by Ro/Ri=4.4 (FIG. 3), and the case where the center block 7 and the ring-shape center block 10 are removed is expressed by Ro/Ri=2 (FIG. 4). Thus a orientation rate in Y direction is adjustable by changing expansion ratio with the center block 7, 10, so that various types of cylindrical bodies can be molded by a single unit of expansion die 1.

And when the center block 10 is removed, the flow passage width W′ in front of the inlet portion 6 i is more than five times as large as the flow passage width Wi of the inlet portion 6 i. Therefore the rubber or synthetic resin mixed with short fiber is oriented at random in respective directions X, Y, Z without disproportioning orientation rate of the short fiber in one direction in front of the inlet portion 6 i so that disadvantageous factor for short fiber orientation in the mold opening 6 is eliminated.

Although the center block 7 is provided to the outer die 3 in the above described expansion die 1, it may be provided to the inner die 5 and may be provided to both of the outer die 3 and the inner die 5.

The orientation rate of the cylindrical body of rubber including short fiber that is molded using this expansion die 1 is examined. The orientation rate is obtained in such way that specimens of 37 mm diameter are stamped out after the cylindrical body is vulcanized, the specimens are immersed in toluene for 50 hours at the room temperature, swelling degree of the cylindrical body is measured in longitudinal direction (X direction), circumferential direction (Y direction), and radius direction (Z direction), and percentage value of inverse number thereof is orientation rate. This is because mechanical performance of tensile strength increases and deformation due to swelling is restrained, when short fibers are much oriented.

[Test 1]

Rubber mixed with short fiber of aspect ratio 110 is extruded from the expansion die 1 attached with the center block 7 (Ro/Ri=7.5) to mold the cylindrical body including short fiber. The test is conducted under the condition that mandrel diameter of the extruder is ø 90 mm, revolution rate of the extrusion screw is 10 min⁻¹, temperature of the die is 100° C. As for rubber composition, EP rubber is 100 weight, carbon black is 70 weight, oil is 20 weight, short fiber is 20 weight, and others are 12 weight. And when orientation rate of the vulcanized rubber including sort fiber is examined, Y direction is 89.6%, X direction is 6.2%, Z direction is 4.2%.

Next, when the cylindrical body of rubber including short fiber is molded with the ring-shaped center block 10 while the center block 10 being attached (Ro/Ri=4.4), and the orientation rate of vulcanized rubber including short fiber is examined, Y direction is 73.1%, X direction is 15.3%, Z direction is 11.6%.

And when the cylindrical body of rubber including short fiber is molded while the center block 7 and the ring-shape center block 10 being removed (Ro/Ri=2) and orientation rate of vulcanized rubber including short fiber is similarly examined, Y direction is 44.5%, X direction is 32.7%, Z direction is 22.8%. The results of this test are shown in Table 1.

TABLE 1 Die flow passage Expansion Fiber width ratio Variable Orientation rate aspect Wi Wc Wo Ro/Ri Wo/Wi Wo/Wi Wo/Wc Y X Z ratio 3 3 3 7.5 1 1 1 89.6 6.2 4.2 110 3 3 3 4.4 1 1 1 73.1 15.3 11.6 110 3 3 3 2 1 1 1 44.5 32.7 22.8 110

[Test 2]

Rubber mixed with short fiber of aspect ratio 35 is extruded from the expansion die 1 attached with the center block 7 (Ro/Ri 7.5) to mold the cylindrical body including short fiber. The condition is the same with that of Test 1. And the orientation rate of the short fiber included vulcanized rubber that is obtained by vulcanizing the molded cylindrical body is examined, Y direction is 78.2%, X direction is 13.6%, Z direction is 8.2%.

Next, when the cylindrical body of rubber including short fiber is molded with the ring-shaped center block 10 while being attached (Ro/Ri=4.4) and the orientation rate of vulcanized rubber including short fiber is similarly examined, Y direction is 67.7%, X direction is 19.4%, Z direction is 12.9%.

And when the cylindrical body of rubber including short fiber is molded with the center block 7 and the ring-shape center block 10 being removed (Ro/Ri=2) and orientation rate of vulcanized rubber including short fiber is similarly examined, Y direction is 38.2%, X direction is 31.9%, Z direction is 29.9%. The results of this test are shown in Table 2.

TABLE 2 Die flow passage Expansion Fiber width ratio Variable Orientation rate aspect Wi Wc Wo Ro/Ri Wo/Wi Wo/Wi Wo/Wc Y X Z ratio 3 3 3 7.5 1 1 1 78.2 13.6 8.2 35 3 3 3 4.4 1 1 1 67.7 19.4 12.9 35 3 3 3 2 1 1 1 38.2 31.9 29.9 35

Results of Test 1 and Test 2 show that Y direction orientation rate described in Tables 1 and 2 can be realized by one pass from a random state in such that short fiber is uniformly oriented to directions of X Y Z respectively (approximately 33.3% of orientation rate to respective directions), and the cylindrical body to satisfy mechanical property in Y direction remarkably efficiently compared with the conventional calendar molding can be molded.

And compared with Y direction orientation rate and value of arctangent of expansion ratio in FIG. 5, either test result of short fiber aspect ratio of 35 and 110 show a close value to arctangent of expansion ratio. Therefore, it is realized that expansion ratio Ro/Ri may be set within a scope of 2≦Ro/Ri≦8 as a guide for efficiently orienting short fiber in Y direction.

[Test 3]

Rubber mixed with respective short fiber of aspect ratio 2 to 5, 370, 500 is extruded from the expansion die 1 attached with the center block 7 (Ro/Ri=7.5) to mold the cylindrical body including short fiber. In the cylindrical body including short fiber of aspect ratio 370 and 500 is molded. Then in the cylindrical body including short fiber of aspect ratio of 370 and 500, short fiber is in the state of lump and it is not proper as a rubber material. That means, if short fiber exists in the state of lump in the rubber material, deformation occurs during vulcanization and strength distribution becomes uneven. And examined orientation rate of the vulcanized rubber including short fiber that is obtained by vulcanizing the cylindrical body including short fiber having aspect ratio 2 to 5, Y direction is 36.2%, X direction is 37.1%, and Z direction is 26.7%, and the orientation rate is higher in X direction than in Y direction. The results of this test 3 are shown in Table 3.

TABLE 3 Die flow passage Expansion Fiber width ratio Variable Orientation rate aspect Wi Wc Wo Ro/Ri Wo/Wi Wo/Wi Wo/Wc Y X Z ratio 3 3 3 7.5 1 1 1 — — — 500 3 3 3 7.5 1 1 1 — — — 370 3 3 3 7.5 1 1 1 36.2 37.1 26.7 2 to 5

Based on the results of the test, if short fiber aspect ratio is out of the scope of 10 to 300, it is found difficult to mold the cylindrical body to satisfy Y direction mechanical property in Y direction.

In the above described embodiment, the taper surface 7 b is formed for restricting the flow passage P in front side of the extrusion direction of the inner surface of the center block. If possible to fully applying pressure to the molding material, a straight surface 7 b′ can be formed as shown in FIG. 6 (a) and a bell-shape surface with diameter gradually expanding in extrusion direction like the surface 7″ in FIG. 6 (b). 

1. A method for manufacturing short fiber oriented rubber or synthetic resin comprising steps of: extruding rubber or synthetic resin mixed with short fiber from a mold opening that is sectionally annular, molding a cylindrical body of the rubber or synthetic resin with the short fiber oriented in a predetermined direction, and passing through the mold opening an expansion portion with a diameter expanding in extrusion direction wherein a flow passage width Wi of an inlet portion of the mold opening, a flow passage width Wo of an outlet portion, and radiuses Ri and Ro of the inlet portion and the outlet portion respectively that are coaxial with the mold opening satisfy a relation of Wo/Wi<Ro/Ri, and orientation of the short fiber is adjusted by adjusting a value of Ro/Ri within a scope of 2≦Ro/Ri≦8 according to mechanical property in circumferential direction of the cylindrical body to be molded.
 2. The method for manufacturing short fiber oriented rubber or synthetic resin according to claim 1, wherein aspect ratio of the short fiber is 10 to
 300. 3. The method for manufacturing short fiber oriented rubber or synthetic resin according to claim 1, wherein the flow passage width of the mold opening is constant.
 4. An expansion die for extrusion molding of an expansion die head for extrusion molding comprising: a mold opening that is sectionally annular and formed with an inner surface of an outer die and an outer surface of an inner die, and an expansion portion whose diameter expands in extrusion direction being provided to the mold opening, wherein a flow passage width Wi of an inlet portion of the mold opening, a flow passage width Wo of an outlet portion, and radiuses Ri and Ro of the inlet portion and the outlet portion respectively that are coaxial with the mold opening have a relation of Wo/Wi<Ro/Ri, and an expansion ratio Ro/Ri is adjusted within a scope of 2≦Ro/Ri≦8 by changing the radius Ri of the inlet portion according to mechanical property in circumferential direction of the cylindrical body to be molded, so that orientation of the short fiber is adjustable.
 5. The expansion die for extrusion molding according to claim 4, wherein a flow passage width of the mold opening keeps constant.
 6. The expansion die for extrusion molding according claim 4, wherein a center block is provided at least one of the outer die and the inner die in such manner that the center block is partially dividable, and the center block is provided with a surface that partially forms the inner surface of the outer die, the outer surface of the inner die, and the mold opening, and the radius Ri of the inlet portion is changeable by attaching, removing, or exchanging the center block.
 7. The expansion die for extrusion molding according to claim 6, wherein a flow passage width W′ in immediate front of the inlet portion after the center block is removed is not less than 5 times of the flow passage width Wi of the inlet portion.
 8. The method for manufacturing short fiber oriented rubber or synthetic resin according to claim 2, wherein the flow passage width of the mold opening is constant.
 9. The expansion die for extrusion molding according claim 5, wherein a center block is provided at least one of the outer die and the inner die in such manner that the center block is partially dividable, and the center block is provided with a surface that partially forms the inner surface of the outer die, the outer surface of the inner die, and the mold opening, and the radius Ri of the inlet portion is changeable by attaching, removing, or exchanging the center block. 